This invention relates in general to analog-to-digital (A/D) converters and, more particularly, to a new architecture for oversampled delta-sigma (.DELTA.--.SIGMA.) A/D conversion of high frequency, narrow band signals using cascaded low order stages with an arbitrary center frequency to obtain high overall order of noise shaping.
Single-loop bandpass delta-sigma (.DELTA.--.SIGMA.) modulation is described by R. Schreier and M. Snelgrove in "Bandpass Sigma Delta Modulation," Electronics Letters, Vol. 25, Nov. 9, 1989, pp. 1560-1561. Z. X. Zhang, G. C. Temes and Z. Czamul in "Bandpass .DELTA..SIGMA. A/D Converter Using Two-Path Multibit Structure", Electronics Letters, Vol. 27, Oct. 24, 1991, pp. 2008-2009, citing Schreier et al., supra, describe a modification of the basic structure using N-path switched capacitor circuits. Subsequently, R. Schreier, G. C. Temes, A. G. Yesilyurt, Z. X. Zhang, Z. Czarnul, and A. Hairapetian in "Multibit Bandpass Delta-Sigma Modulators Using N-Path Structures", IEEE International Symposium on Circuits and Systems, May 1992, pp. 593-596, describe simulation results of two switched-capacitor circuits for use in a multi-bit bandpass sigma-delta modulator.
Typically, cascaded low-order delta-sigma stages have been used to implement higher order delta-sigma modulators for A/D conversion of low-pass signals. See, for example, D. B. Ribner, "A Comparison of Modulator Networks for High-Order Oversampled Sigma-Delta Analog-to-Digital Converters," IEEE Trans. Circuits and Systems, Vol. CAS-38, No. 2, pp. 145-159, Feb. 1991. Those cascades comprise combinations of first and second-order individual stages and thereby avoid the stability problem prone to single-loop modulators of third-order and higher. See also D. B. Ribner U.S. Pat. Nos. 5,084,702, 5,103,229, and 5,148,166, all assigned to the present assignee, for further background on plural-order sigma-delta analog-to-digital converters. U.S. Pat. Nos. 5,084,702, 5,103,229 and 5,148,166 are hereby incorporated by reference.
U.S. Pat. No. 5,283,578, issued Feb. 1, 1994, by D. B. Ribner and D. H. K. Hoe, assigned to the present assignee and herein incorporated by reference, provides an architecture for multistage delta-sigma (.DELTA.--.SIGMA.) bandpass modulators that provide band-reject noise shaping at one fourth the sampling frequency F.sub.s. Although the foregoing multistage architecture, advantageously overcomes many of the problems of previous single stage high order architectures, such as stability, component spread and design complexity, the center frequency of the bandpass modulators therein is limited to one fourth of the sampling frequency F.sub.s.